Image copyrightAlexander BaroshinImage caption
Ildar Dadin was sentenced in December for taking part in repeated illegal street protests

Russian officials are investigating claims that a well-known opposition activist has been repeatedly beaten and threatened in prison.

Ildar Dadin is serving two and a half years for a series of street protests.

He told his lawyer that he had been strung up by his handcuffed wrists, had his head shoved down a toilet and been threatened with rape and death.

Russia's prison service says the claims are untrue but will investigate if the activist complains formally.

Unusually, the allegations made the headlines on state-controlled television on Tuesday after Kremlin spokesman Dmitry Peskov told reporters that the case merited "the closest attention" and that President Vladimir Putin would be informed.

Image copyrightElizaveta Vereykina Image caption
A protester outside the justice ministry held a sign saying: "They hang Dadin by his handcuffs and 12 men beat him for taking to the streets with a protest banner. Torture is banned under Article 21 of the Russian constitution"

Ildar Dadin, 34, says his problems began after he was transferred to Prison Number Seven in the Karelia region of north-western Russia in mid-September and sent directly to a punishment cell.

Now out of isolation, Dadin has dictated a letter to his wife via his lawyer, in which he says he was beaten and kicked repeatedly by up to a dozen prison staff.

One day, he alleges, he was hung from his wrists for about half an hour causing "extreme pain".

"They then took off my pants and said that they would bring in another prisoner who would rape me if I didn't agree to end my hunger strike," Ildar Dadin writes.

He had stopped eating in protest at being held in a punishment cell.

Russia's FSIN prison service told the BBC that Mr Dadin was put in isolation for what a spokeswoman called "violations of the detention regime", but insisted that his claims of beating and torture were false.

"There's no way illegal methods were used, especially for such a well-known character," Kristina Belousova told the BBC by telephone.

"It's not the first time we've seen this, by people trying to attract maximum attention," the spokeswoman added.

Image copyrightAlexander BaroshinImage caption
Dadin's lawyer described his client as frightened and subdued

Ildar Dadin was sentenced in December 2015, under Article 212.1 - a controversial law that makes repeat violations of Russia's strict laws on street protests a criminal offence.

"We didn't expect this to happen. It's awful," lawyer Alexei Liptser told the BBC, after visiting his client on Monday and transcribing his account.

"He is frightened and subdued," the lawyer added.

As the allegations of abuse spread on social media, and reached the Kremlin, Russia's human rights ombudswoman ordered a regional representative to visit the prison and said she was taking the case under her "personal control".

In a dramatic conclusion to his letter, Ildar Dadin tells his wife "I am not afraid of dying. I'm most afraid of not being able to tolerate the torture."